There are various forms of equipment for measuring the position of a motor vehicle wheel, typically comprising a detector device and a wheel adaptor with which the detector device can be fixed to the motor vehicle wheel to be measured, in aligned relationship with respect to the axis of the wheel.
Wheel position measuring apparatuses of that kind which may be optical measuring apparatuses and which may use projectors and measuring mirrors as the detector devices are fixed to the respective vehicle wheel whose position is to be measured, by means of the wheel adaptor. Other forms of such apparatuses operate by measurement using mechanical and electromagnetic systems.
However, before measurement operations can be carried out, it is necessary to perform an operation such as to compensate for any out-of-truth of the wheel and more particularly the rim portion thereof. For that purpose, an optical or electronic sensor is mounted on the wheel which is in a condition of being lifted off the ground, by means of the wheel adaptor, and the, when the motor vehicle wheel is rotated, any deflection movement due to possible out-of-trust or run-out of the wheel rim is measured in two planes (in respect of camber and track), such deflection movement occurring as a sinusoidal movement. However, when the wheel is lowered on to the ground again after that measuring operation, it is found that, due to the nature of the wheel suspension and the mounting thereof to the motor vehicle, it experiences an angular displacement relative to the reference angle position (which can also be referred to as the zero point) which was ascertained in the operation of compensating for any out-of-trust of the wheel rim, when the wheel was in a lifted condition. That angular displacement then appears in the form of a measurement error in the subsequent measurements in respect of the wheel position.
Measurements in respect of steering swivel pin angle and sum angle are effected by applying steering lock. With some types of vehicles and in particular modern sportscars, that operation can involve some difficulties as spoilers, downwardly extended guard plates and the like are in the way of a pivotal movement, in particular of a detector which is arranged as a front sensor on the motor vehicle wheel, on an arm which projects forwardly of the vehicle. It frequently happens also that pivotal movement of that kind is impeded by obstacles on the ground. Those problems and difficulties arise in particular in the case of motor vehicles which have a low ride height so that their body has only a small amount of ground clearance.